3.04: Viruses and Prokaryotes

Bacteria and archaeans have which characteristic in common?

Both are prokaryotic.

Which group contains only prokaryotes?

Domains Archaea and Bacteria

Do viruses belong to one of the domains of life?

No, they are not considered to be alive.

Although Domain Bacteria is very diverse

bacteria share some basic characteristics.

Many bacteria reproduce using a process called

binary fission. In binary fission, the bacterial chromosome replicates, and the bacterium pinches into two separate cells. Each new bacterium receives a copy of the single chromosome

Viruses are not

cells - They do not take in energy from the environment. - They do not grow. - They do not have a metabolism. - They do, however, reproduce, but they can't do it alone. A virus injects its DNA or RNA into the cells of living organisms. The living cell then makes many new copies of the virus.

Viruses are

nonliving particles that cause infection in many organisms.

While both domains are made of prokaryotic cells, the two groups differ in several fundamental ways.

- Archaean cells are protected by unique cell walls made up of lipids unlike those found in either bacteria or eukaryotes. - Unlike bacteria, archaean cell walls do not contain peptidoglycan. - Archaeans and eukaryotes also have similar proteins in their ribosomes. These proteins are different from those found in bacterial ribosomes. - Based on these characteristics, as well as DNA and other molecular evidence, scientists have determined that archaeans are more closely related to eukaryotes than they are to bacteria.

Despite their differences, bacteria share some basic traits that set them apart from either eukaryotes or archaeans.

- Bacteria are prokaryotes. - They are all single-cell organisms lacking a nucleus. - Their DNA is one single, circular chromosome. - Bacteria are surrounded by a strong cell wall made of a molecule called peptidoglycan. - Bacteria have distinct amino acids in some of the proteins they produce.

One way of classifying bacteria into groups considers how they obtain energy.

Autotrophic bacteria make their own energy, either through photosynthesis or through chemosynthesis, a process in which bacteria use chemical elements such as sulfur and nitrogen to generate energy. Heterotrophic bacteria break down organic matter in the environment-in other words, they must break down material produced by other living organisms to produce their own energy.

Why are bacteria and archaea considered to be living organisms?

Bacteria and Archaea are living organisms because they are cellular, metabolize, and are able to reproduce independently.

The prokaryotic domains are Archaea and Bacteria. Viruses are not living things.

Two of the three domains of life include only prokaryotic organisms. These domains—Bacteriaand Archaea—are made up of organisms as different from each other as they are from you.While they are microscopic, the organisms in these two domains make up the vast majority of life on earth, and they play many significant roles in human health, the environment, and industry.Viruses are not living, as they are not cellular.They need living cells to help them reproduce.

Why are viruses not considered living organisms?

Viruses are nonliving because they are not cellular. Viruses cannot manufacture their own proteins, and must rely on the living cell of another organism to reproduce.

Viruses reproduce inside the cells of living organisms.

Viruses cannot manufacture their own proteins, and they must rely on the living cell of another organism to reproduce.

Some archaeans live in extreme habitats, but many live in places such as soil and seawater.

When scientists first began learning about archaeans, they found them in extreme habitats, such as the extremely salty water of the Great Salt Lake in Utah. Since then, scientists have learned that most archaeans share some DNA sequences, and they have used these sequences to help search for archaeans in other places. It turns out that while archaeans do live in some unusual habitats, many live in more ordinary places, too, such as soil.

Bacteria make up one large, prokaryotic domain.

Scientists have divided Domain Bacteria into several broad groups

One species of archaean made possible much of the DNA revolution.

The archaean species Thermus aquaticus, which lives in hot springs at temperatures near the boiling point of water, has become very important to molecular biologists around the world. This species produces an enzyme called polymerase. Polymerase is an enzyme that all species rely on for DNA replication. Most polymerase enzymes cannot tolerate high temperatures, but the polymerase found in T. aquaticus can. For this reason, scientists use this polymerase in the chemical process PCR (polymerase chain reaction). PCR produces thousands of copies of a DNA strand in a short time, and these copies have many uses in biotechnology. The process requires both high temperatures and a polymerase that won't degrade at those temperatures. The discovery of the polymerase in T. aquaticus, called Taq polymerase, opened the door to many advances in DNA research.

Domain Archaea includes another broad group of prokaryotes.

Domains Archaea and Bacteria split apart from each other all the way at the base of the tree. This split shows that scientists believe the two groups, or lineages, split apart from each other very early in the evolution of life.

On earth, there are vastly more prokaryotes than eukaryotes.

If you were asked to describe the diversity of life on earth, you'd probably think of all the eukaryotes you know: birds, fish, clams, trees, flowers, and insects. But, eukaryotes-organisms made of cells with a nucleus and other membrane-bound organelles-make up just a fraction of the species alive on earth today. The vast majority of life on earth is made up of prokaryotes, the single-cell organisms that are too small to see without a microscope. Two of the three domains of life include only prokaryotic organisms. These domains-Bacteria and Archaea-are made up of organisms as different from each other as they are from you. You might not guess it on the surface, but a closer look at the molecular and genetic makeup of the organisms in these two domains shows just how unique each is.

In the lysogenic cycle, the virus's genetic material is copied each time the cell divides.

In some viruses, reproduction also involves the lysogenic cycle. In the lysogenic cycle, a virus injects its DNA into a cell, and the DNA becomes a part of that cell's genome. When the host cell replicates its DNA and divides, it also replicates the viral DNA. The viral DNA will be found in all new cells being created. Depending on the virus, the viral DNA may not become active for many years. It is almost as if the viral DNA is asleep. Eventually, when the conditions are right, the viral DNA in the genome is turned on. The cell switches over to the lytic cycle and starts building viruses. Eventually the cell bursts and dies, and the virus particles float away. There are viruses that infect all types of cells. As scientists work to develop vacines, new resistant viral strains develop as virual DNA mutates.

Bacteria can be helpful as well as harmful.

People tend to associate bacteria with illness. They're not wrong:Streptococcal bacteria destroy red blood cells, and the bacteria that cause the disease tuberculosis break down lung tissue and use it as a source of energy. Here are some significant human diseases caused by various species of bacteria. Have you heard of any of them? They frequently make news headlines. However, it's important to know that not all bacteria cause disease. Many bacteria play helpful roles in ecosystems, food processing, and your body's digestive system. Bacteria help turn milk into yogurt and cheese. Other bacteria break down dead or decaying organisms on the forest floor. And if it weren't for bacteria, you'd have a hard time digesting many foods. Bacteria that live in your large intestine help break down many types of carbohydrates during digestion. The list of beneficial bacteria is very long.

Have you ever had chicken pox? How about the flu? Or the common cold? Each of these illnesses-and many others-is caused by

a virus. Viruses are nonliving particles that are made up of a protein coat surrounding a segment of either DNA or RNA.

Viruses reproduce in one of two different ways:

the lytic cycle and the lysogenic cycle. In the lytic cycle, a virus infects a cell by injecting its DNA (or RNA) into the cell. The cell's DNA replication machinery-enzymes and other proteins-replicate the viral genes and produce the proteins required to make the capsid, or protein coat of the virus. The viral DNA directs the living cell to assemble new viral particles.Ultimately, the cell bursts open, releasing new viruses. The new viruses can go on and infect new cells.